1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a method for manufacturing the semiconductor device.
In this specification, the semiconductor device refers to all devices that can function by utilizing semiconductor characteristics. A transistor in this specification is a semiconductor device, and an electrooptic device, a semiconductor circuit, and an electronic device including the transistor are all semiconductor devices.
2. Description of the Related Art
Transistors used for most flat panel displays typified by a liquid crystal display device and a light-emitting display device are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Further, transistors formed using such silicon semiconductors are used in integrated circuits (ICs) and the like.
Attention has been directed to a technique in which, instead of the above silicon semiconductors, metal oxides exhibiting semiconductor characteristics are used for transistors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.
For example, a technique is disclosed in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like of a pixel of a display device (see Patent Documents 1 and 2).
Further, a technique is disclosed in which in a transistor including an oxide semiconductor, a highly conductive oxide semiconductor containing nitrogen is provided as buffer layers between a source region and a source electrode and between a drain region and a drain electrode, and thereby the contact resistance between the oxide semiconductor and the source electrode and between the oxide semiconductor and the drain electrode is reduced (see Patent Document 3).
Further, as a method for forming a source region and a drain region of a transistor including an oxide semiconductor in a self-aligned manner, a method is disclosed in which a surface of the oxide semiconductor is exposed and argon plasma treatment is performed, and thereby the resistivity of the exposed portion of the oxide semiconductor is reduced (see Non-Patent Document 1).
In this method, however, since the surface of the oxide semiconductor is exposed and argon plasma treatment is performed, portions of the oxide semiconductor to be the source region and the drain region are also etched, leading to decrease in the thicknesses of the source region and the drain region (see FIG. 8 in Non-Patent Document 1). As a result, the resistance of the source region and the drain region is increased, and in addition, defective products are produced with higher probability owing to overetching due to the decrease in thickness.
This phenomenon is remarkable in the case where the atomic radius of an ion species used for the plasma treatment on the oxide semiconductor is large.
Such a problem does not arise if an oxide semiconductor layer has a sufficient thickness. However, when the channel length is less than or equal to 200 nm, it is necessary that the thickness of a portion of the oxide semiconductor layer which serves as a channel be less than or equal to 20 nm, preferably less than or equal to 10 nm, for prevention of a short-channel effect. The above plasma treatment is not suitable in the case where such a thin oxide semiconductor layer is used.